CATHODE PHENOMENA IN PLASMA THRUSTERS

For the optimization of cathodes of steady-state plasma accelerators, an extensive experimental program has been carried out at IRS to inves tigate the erosion mechanisms. On different thruster types and with an additional fundamental experiment, the influences of geometry, chambe r pressure, amount of ThO2, cathode temperature, power, and gas specie s were explored. Propellants Ar, H-2, N2 were used which had been puri fied with a special filter to remove residual water and oxygen. The er osion rates during the ignition phase as well as during continuous ope ration were measured. The temporal evolution of the cathode erosion ra te was measured by surface layer activation. At high current densities within the cathode, severe problems were encountered with all thruste r geometries. The cathodes developed cracks and started to melt. To cl arify the reason for this behavior, some cathodes were investigated me tallurgically.